We don’t normally think about males running out of sperm, however, sperm isn’t unlimited and can be costly for a body to make. In polygynous species where males mate with multiple females, it’s not uncommon for males to run low on reserves and produce fewer sperm with each successive copulation with a female.

Therefore, it makes sense for females to preferentially choose to mate with males who haven’t recently mated with other females.

In a recent study published in Ethology, Scarponi and colleagues from Carleton University in Canada used the Trinidadian guppy Poecilia reticulata to see whether females could identify and avoid mating with males who had recently mated with other females.

Male guppies attempt to woo females through courtship displays, but when that doesn’t work they tend to just go for it. As there is intense competition between males for females, they try to copulate with females really at any chance they get. After just a single copulation, a male depletes around 92% of his sperm stores and replenishing these stores takes three days. However, this doesn’t stop him from keeping trying to copulate with other females. Indeed, in the wild, males may attempt copulations with females around once a minute.

To test whether female guppies preferred males who hadn’t recently copulated with another female, the researchers placed a ‘focal’ female in a central part of a fish tank, with two males at either end of the tank to her held in separate compartments. One of the males was alone (the non-sperm-depleted male) while the other male had a female in the tank with them. As all it takes for a male guppy to copulate is the presence of a female guppy, in all cases the male guppies mated with the female in the tank with them. These males were therefore much more sperm-depleted compared to the other males who hadn’t just copulated with females. The focal female could therefore see each male, but not come into physical contact with them. The researchers looked to see which male the female guppy swam towards and seemed to want to spend time with.

In a second choice test, the researchers gave females the chance to choose between males in a similar set-up, but where they could come into direct contact with them.

The researchers found that in both choice tests the females avoided the males that had recently been seen sexually interacting with another female. Instead they preferred to mate with the virgin males who would not be sperm-depleted.

Female guppies prefer to mate with a male who they haven’t just seen copulating with a female. Photo: tartaruga33

Which cues do the females use to decide not to mate with a male? To answer this question the researchers are now carrying out a follow-up experiment to determine whether the females are using the males’ behaviour, or instead an indirect cue such as a chemical a male might give off after mating with another female. Watch this space for the answer!

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